x86/alt: Clean up the assembly used to generate alternatives

x86/alt: Clean up the assembly used to generate alternatives

 * On the C side, switch to using local lables rather than hardcoded numbers.
 * Rename parameters and lables to be consistent with alt_instr names, and
   consistent between the the C and asm versions.
 * On the asm side, factor some expressions out into macros to aid clarity.
 * Consistently declare section attributes.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>